Methods of washing cellulose-rich solids from biomass fractionation to reduce lignin and ash content

ABSTRACT

The present invention provides a process for fractionating lignocellulosic biomass, comprising: digesting a biomass feedstock in the presence of a solvent for lignin, an acid, and water, to produce cellulose-rich solids; separating and washing the cellulose-rich solids with a wash solvent; washing the cellulose-rich solids with water, to generate washed cellulose-rich solids and a wash liquor comprising fines, wherein the wash liquor is introduced to or in contact with a classifier to remove the fines; and separating the fines and recycling the remaining water. The classifier may include a screen with mesh size in the range of 10 to 500, such as 200. The washed cellulose-rich solids will typically have a lower Kappa number (lignin content) and ash content compared to cellulose-rich solids from a process without a classifier that removes fines.

PRIORITY DATA

This patent application is a non-provisional application claiming priority to U.S. Provisional Patent App. No. 61/905,938, filed Nov. 19, 2013, which is hereby incorporated by reference herein.

FIELD

The present invention generally relates to fractionation processes for converting biomass into fermentable sugars, cellulose, and lignin, and for processes and apparatus to recover the lignin.

BACKGROUND

Biomass refining (or biorefining) is becoming more prevalent in industry. Cellulose fibers and sugars, hemicellulose sugars, lignin, syngas, and derivatives of these intermediates are being used by many companies for chemical and fuel production. Indeed, we now are observing the commercialization of integrated biorefineries that are capable of processing incoming biomass much the same as petroleum refineries now process crude oil. Underutilized lignocellulosic biomass feedstocks have the potential to be much cheaper than petroleum, on a carbon basis, as well as much better from an environmental life-cycle standpoint.

Lignocellulosic biomass is the most abundant renewable material on the planet and has long been recognized as a potential feedstock for producing chemicals, fuels, and materials. Lignocellulosic biomass normally comprises primarily cellulose, hemicellulose, and lignin. Cellulose and hemicellulose are natural polymers of sugars, and lignin is an aromatic/aliphatic hydrocarbon polymer reinforcing the entire biomass network. Some forms of biomass (e.g., recycled materials) do not contain hemicellulose.

There are many reasons why it would be beneficial to process biomass in a way that effectively separates the major fractions (cellulose, hemicellulose, and lignin) from each other. Cellulose from biomass can be used in industrial cellulose applications directly, such as to make paper or other pulp-derived products. The cellulose can also be subjected to further processing to either modify the cellulose in some way or convert it into glucose. Hemicellulose sugars can be fermented to a variety of products, such as ethanol, or converted to other chemicals. Lignin from biomass has value as a solid fuel and also as an energy feedstock to produce liquid fuels, synthesis gas, or hydrogen; and as an intermediate to make a variety of polymeric compounds. Additionally, minor components such as proteins or rare sugars can be extracted and purified for specialty applications.

In light of this objective, a major shortcoming of previous process technologies is that one or two of the major components can be economically recovered in high yields, but not all three. Either the third component is sacrificially degraded in an effort to produce the other two components, or incomplete fractionation is accomplished. An important example is traditional biomass pulping (to produce paper and related goods). Cellulose is recovered in high yields, but lignin is primarily consumed by oxidation and hemicellulose sugars are mostly degraded. Approximately half of the starting biomass is essentially wasted in this manufacturing process. State-of-the-art biomass-pretreatment approaches typically can produce high yields of hemicellulose sugars but suffer from moderate cellulose and lignin yields.

There are several possible pathways to convert biomass into intermediates. One thermochemical pathway converts the feedstock into syngas (CO and H₂) through gasification or partial oxidation. Another thermochemical pathway converts biomass into liquid bio-oils through pyrolysis and separation. These are both high-temperature processes that intentionally destroy sugars in biomass.

Sugars (e.g., glucose and xylose) are desirable platform molecules because they can be fermented to a wide variety of fuels and chemicals, used to grow organisms or produce enzymes, converted catalytically to chemicals, or recovered and sold to the market. To recover sugars from biomass, the cellulose and/or the hemicellulose in the biomass must be hydrolyzed into sugars. This is a difficult task because lignin and hemicelluloses are bound to each other by covalent bonds, and the three components are arranged inside the fiber wall in a complex manner. This recalcitrance explains the natural resistance of woody biomass to decomposition, and explains the difficulty to convert biomass to sugars at high yields.

Fractionation of biomass into its principle components (cellulose, hemicellulose, and lignin) has several advantages. Fractionation of lignocellulosics leads to release of cellulosic fibers and opens the cell wall structure by dissolution of lignin and hemicellulose between the cellulose microfibrils. The fibers become more accessible for hydrolysis by enzymes. When the sugars in lignocellulosics are used as feedstock for fermentation, the process to open up the cell wall structure is often called “pretreatment.” Pretreatment can significantly impact the production cost of lignocellulosic ethanol.

One of the most challenging technical obstacles for cellulose has been its recalcitrance towards hydrolysis for glucose production. Because of the high quantity of enzymes typically required, the enzyme cost can be a tremendous burden on the overall cost to turn cellulose into glucose for fermentation. Cellulose can be made to be reactive by subjecting biomass to severe chemistry, but that would jeopardize not only its integrity for other potential uses but also the yields of hemicellulose and lignin.

Many types of pretreatment have been studied. A common chemical pretreatment process employs a dilute acid, usually sulfuric acid, to hydrolyze and extract hemicellulose sugars and some lignin. A common physical pretreatment process employs steam explosion to mechanically disrupt the cellulose fibers and promote some separation of hemicellulose and lignin. Combinations of chemical and physical pretreatments are possible, such as acid pretreatment coupled with mechanical refining. It is difficult to avoid degradation of sugars. In some cases, severe pretreatments (i.e., high temperature and/or low pH) intentionally dehydrate sugars to furfural, levulinic acid, and related chemicals. Also, in common acidic pretreatment approaches, lignin handling is very problematic because acid-condensed lignin precipitates and forms deposits on surfaces throughout the process.

One type of pretreatment that can overcome many of these disadvantages is called “organosolv” pretreatment. Organosolv refers to the presence of an organic solvent for lignin, which allows the lignin to remain soluble for better lignin handling. Traditionally, organosolv pretreatment or pulping has employed ethanol-water solutions to extract most of the lignin but leave much of the hemicellulose attached to the cellulose. For some market pulps, it is acceptable or desirable to have high hemicellulose content in the pulp. When high sugar yields are desired, however, there is a problem. Traditional ethanol/water pulping cannot give high yields of hemicellulose sugars because the timescale for sufficient hydrolysis of hemicellulose to monomers causes soluble-lignin polymerization and then precipitation back onto cellulose, which negatively impacts both pulp quality as well as cellulose enzymatic digestibility.

An acid catalyst can be introduced into organosolv pretreatment to hydrolyze hemicellulose into monomers while still obtaining the solvent benefit. Conventional organosolv wisdom dictates that high delignification can be achieved, but that a substantial fraction of hemicellulose must be left in the solids because any catalyst added to hydrolyze the hemicellulose will necessarily degrade the sugars (e.g., to furfural) during extraction of residual lignin.

Solvent cooking chemicals have been attempted as an alternative to Kraft or sulfite pulping. The original solvent process is described in U.S. Pat. No. 1,856,567 by Kleinert et al. Groombridge et al. in U.S. Pat. No. 2,060,068 showed that an aqueous solvent with sulfur dioxide is a potent delignifying system to produce cellulose from lignocellulosic material. Three demonstration facilities for ethanol-water (Alcell), alkaline sulfite with anthraquinone and methanol (ASAM), and ethanol-water-sodium hydroxide (Organocell) were operated briefly in the 1990s.

Contrary to conventional wisdom, it has been found that fractionation with a solution of ethanol (or another solvent for lignin), water, and sulfur dioxide (SO₂) can simultaneously achieve several important objectives. The fractionation can be achieved at modest temperatures (e.g., 120-160° C.). The SO₂ can be easily recovered and reused. This process is able to effectively fractionation many biomass species, including softwoods, hardwoods, agricultural residues, and waste biomass. The SO₂ hydrolyzes the hemicelluloses and reduces or eliminates troublesome lignin-based precipitates. The presence of ethanol leads to rapid impregnation of the biomass, so that neither a separate impregnation stage nor size reduction smaller than wood chips are needed, thereby avoiding electricity-consuming sizing operations. The dissolved hemicelluloses are neither dehydrated nor oxidized (Iakovlev, “SO₂-ethanol-water fractionation of lignocellulosics,” Ph.D. Thesis, Aalto Univ., Espoo, Finland, 2011). Cellulose is fully retained in the solid phase and can subsequently be hydrolyzed to glucose. The mixture of hemicellulose monomer sugars and cellulose-derived glucose may be used for production of biofuels and chemicals.

In view of the state of the art, what is desired is to efficiently fractionate any lignocellulosic-based biomass (including, in particular, softwoods) into its primary components so that each can be used in potentially distinct processes. While not all commercial products require pure forms of cellulose, hemicellulose, or lignin, a platform biorefinery technology that enables processing flexibility in downstream optimization of product mix, is particularly desirable. An especially flexible fractionation technique would not only separate most of the hemicellulose and lignin from the cellulose, but also render the cellulose highly reactive to cellulase enzymes for the manufacture of fermentable glucose.

The AVAP® fractionation process developed by American Process, Inc. and its affiliates is able to economically accomplish these objectives. Improvements are still desired in the area of washing of pulp to reduce lignin and ash content.

SUMMARY

The present invention addresses the aforementioned needs in the art.

In some variations, the invention provides a process for fractionating lignocellulosic biomass, the process comprising:

(a) digesting a lignocellulosic biomass feedstock under effective conditions in the presence of a solvent for lignin, an acid or acid precursor, and water, to produce cellulose-rich solids in a digestor liquor;

(b) separating the cellulose-rich solids from the digestor liquor and washing the cellulose-rich solids with a first wash liquid comprising a wash solvent for lignin, to generate first washed cellulose-rich solids;

(c) washing the first washed cellulose-rich solids with a second wash liquid comprising water, to generate second washed cellulose-rich solids and a wash liquor comprising fines, wherein the wash liquor is introduced to or in contact with a classifier to remove at least a portion of the fines in a liquid fines-containing stream;

(d) recovering the second washed cellulose-rich solids; and

(e) optionally separating the fines from the fines-containing stream and recycling water contained in the fines-containing stream back to step (c).

The lignocellulosic biomass feedstock is a hardwood or an annual plant or agricultural residue, in some embodiments. The solvent for lignin may be ethanol, and the wash solvent for lignin may be the same (e.g., ethanol) or different. The acid or acid precursor is preferably sulfur dioxide.

In some embodiments, the classifier comprises a screen with mesh size in the range of 10 to 500, such as a range of 100 to 325 or 150 to 250. In certain embodiments, the classifier comprises a screen with mesh size of 200. The classifier may also comprise a centrifuge or other separation device.

In some embodiments, during step (b) and/or step (c), a disperser is utilized to liberate the fines from the second washed cellulose-rich solids. During step (c), a portion of the fines contained in the wash liquor are removed into the liquid fines-containing stream. The portion of fines removed may be at least 50%, at least 75%, or at least 95% of the fines contained in the wash liquor removed into the liquid fines-containing stream.

In some embodiments, during step (b) and/or step (c), one or more additives are introduced to remove minerals remaining in the first washed cellulose-rich solids and/or the second washed cellulose-rich solids.

The second washed cellulose-rich solids will typically have a lower Kappa number compared to cellulose-rich solids from an otherwise-identical process without a classifier to remove at least a portion of the fines. In some embodiments, the second washed cellulose-rich solids have a lower ash content compared to cellulose-rich solids from an otherwise-identical process without a classifier to remove at least a portion of the fines. In some of these embodiments, the second washed cellulose-rich solids have a lower hemicellulose content compared to cellulose-rich solids from an otherwise-identical process without a classifier to remove at least a portion of the fines.

For example, the second washed cellulose-rich solids may contain about 75% or more cellulose, about 7 wt % or less lignin, about 5 wt % or less hemicellulose, and about 10 wt % or less ash. In certain embodiments, the second washed cellulose-rich solids contain about 80% or more cellulose, about 3 wt % or less lignin, about 5 wt % or less hemicellulose, and about 8 wt % or less ash.

The process may be continuous or semi-continuous, or batch. In some embodiments, steps (a) and (b) are conducted countercurrently. In some embodiments, steps (a)-(c) are conducted countercurrently. In certain embodiments, the process is batch or semi-continuous, wherein step (b) and/or step (c) is conducted in simulated countercurrent fashion, and wherein multiple wash streams are generated.

In some embodiments, the process further comprises hydrolyzing the second washed cellulose-rich solids to produce glucose. In some embodiments, the process further comprises feeding the second washed cellulose-rich solids to a pulping operation.

The process may further include separating and recycling unreacted acid or acid precursor from the digestor liquor. In some embodiments, the process further comprises further treating the digestor liquor to generate fermentable sugars.

Some variations provide a method of separating fines from cellulose-rich solids, the method comprising:

(a) obtaining a biomass digestor liquor comprising cellulose-rich solids;

(b) separating the cellulose-rich solids from the digestor liquor and washing the cellulose-rich solids with a first wash liquid, to generate first washed cellulose-rich solids;

(c) washing the first washed cellulose-rich solids with a second wash liquid, to generate second washed cellulose-rich solids and a wash liquor comprising fines, wherein the wash liquor is introduced to or in contact with a classifier to remove at least a portion of the fines in a liquid fines-containing stream;

(d) recovering the second washed cellulose-rich solids; and

(e) optionally separating the fines from the fines-containing stream and recycling water contained in the fines-containing stream back to step (c).

Some variations provide a method of separating fines from cellulose-rich solids, the method comprising:

(a) obtaining a biomass digestor liquor comprising cellulose-rich solids;

(b) separating the cellulose-rich solids from the digestor liquor and washing the cellulose-rich solids with a wash liquid, to generate washed cellulose-rich solids and a wash liquor comprising fines, wherein the wash liquor is introduced to or in contact with a classifier to remove at least a portion of the fines in a liquid fines-containing stream;

(c) recovering the second washed cellulose-rich solids; and

(d) optionally separating the fines from the fines-containing stream and recycling water contained in the fines-containing stream back to step (b).

In some embodiments, the classifier comprises a screen with mesh size in the range of 10 to 500, such as 100 to 325 or 150 to 250 (e.g., 200). In some embodiments, a disperser is utilized to liberate the fines from the washed cellulose-rich solids. At least 50%, 75%, or 95% of the fines contained in the wash liquor may be removed into the liquid fines-containing stream.

Optionally, one or more additives are introduced to remove minerals remaining in the washed cellulose-rich solids.

In some embodiments, the washed cellulose-rich solids have a lower Kappa number compared to cellulose-rich solids from an otherwise-identical process without a classifier to remove at least a portion of the fines. In some embodiments, the second washed cellulose-rich solids have a lower ash content compared to cellulose-rich solids from an otherwise-identical process without a classifier to remove at least a portion of the fines. In some embodiments, the second washed cellulose-rich solids have a lower hemicellulose content compared to cellulose-rich solids from an otherwise-identical process without a classifier to remove at least a portion of the fines.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a process of fractionating lignocellulosic biomass, in some embodiments of the invention.

FIG. 2 shows photographs of fibers and fines isolated from the pulp with a Britt jar assembly, in some embodiments.

FIG. 3 summarizes an isolation process of fines from washing water, according to certain embodiments of the invention.

FIG. 4 shows exemplary SEM images of pulp produced from sugarcane straw by AVAP® technology.

FIG. 5 shows exemplary SEM images of fines isolated from the pulp (shown in FIG. 4) during a water-washing stage, according to some embodiments.

FIG. 6 is a schematic representation of certain improved washing procedures, in preferred embodiments.

These drawings are exemplary in nature and should not be construed to limit the invention in any way.

DETAILED DESCRIPTION OF SOME EMBODIMENTS

This description will enable one skilled in the art to make and use the invention, and it describes several embodiments, adaptations, variations, alternatives, and uses of the invention. These and other embodiments, features, and advantages of the present invention will become more apparent to those skilled in the art when taken with reference to the following detailed description of the invention in conjunction with any accompanying drawings.

As used in this specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly indicates otherwise. For example, reference to “unit” also includes a plurality of units (e.g., reactors or vessels). Unless defined otherwise, all technical and scientific terms used herein have the same meaning as is commonly understood by one of ordinary skill in the art to which this invention belongs. All composition numbers and ranges based on percentages are weight percentages, unless indicated otherwise. All ranges of numbers or conditions are meant to encompass any specific value contained within the range, rounded to any suitable decimal point.

Unless otherwise indicated, all numbers expressing parameters, reaction conditions, concentrations of components, and so forth used in the specification and claims are to be understood as being modified in all instances by the term “about.” Accordingly, unless indicated to the contrary, the numerical parameters set forth in the following specification and attached claims are approximations that may vary depending at least upon a specific analytical technique.

The term “comprising,” which is synonymous with “including,” “containing,” or “characterized by” is inclusive or open-ended and does not exclude additional, unrecited elements or method steps. “Comprising” is a term of art used in claim language which means that the named claim elements are essential, but other claim elements may be added and still form a construct within the scope of the claim.

As used herein, the phase “consisting of” excludes any element, step, or ingredient not specified in the claim. When the phrase “consists of” (or variations thereof) appears in a clause of the body of a claim, rather than immediately following the preamble, it limits only the element set forth in that clause; other elements are not excluded from the claim as a whole. As used herein, the phase “consisting essentially of” limits the scope of a claim to the specified elements or method steps, plus those that do not materially affect the basis and novel characteristic(s) of the claimed subject matter.

With respect to the terms “comprising,” “consisting of,” and “consisting essentially of,” where one of these three terms is used herein, the presently disclosed and claimed subject matter may include the use of either of the other two terms. Thus in some embodiments not otherwise explicitly recited, any instance of “comprising” may be replaced by “consisting of” or, alternatively, by “consisting essentially of.”

This disclosure describes processes and apparatus to efficiently fractionate any lignocellulosic-based biomass into its primary major components (cellulose, lignin, and if present, hemicellulose) so that each can be used in potentially distinct processes. An advantage of the process is that it produces cellulose-rich solids while concurrently producing a liquid phase containing a high yield of both hemicellulose sugars and lignin, and low quantities of lignin and hemicellulose degradation products. The flexible fractionation technique enables multiple uses for the products. The washed cellulose is highly reactive to cellulase enzymes for the manufacture of glucose. Other uses for celluloses can be adjusted based on market conditions.

Certain exemplary embodiments of the invention will now be described. These embodiments are not intended to limit the scope of the invention as claimed. The order of steps may be varied, some steps may be omitted, and/or other steps may be added. Reference herein to first step, second step, etc. is for illustration purposes only.

In order to produce cellulose fiber (or pulp) from lignocellulosic biomass, the biomass along with cooking liquor is first cooked in a digestor and then liquid and solid phases are separated. The liquid phase, also called “spent liquor,” mainly includes dissolved biomass substances such as lignin, hemicellulosic and cellulosic sugars in oligomeric and monomeric form, as well as organic acids (acetic acid, uronic acids, formic acid, levulinic acid, lactic acid, etc.), and sugar-degradation products (furfural, hydroxymethylfurfural (HMF), etc.). The spent liquor is typically sent to downstream processes to recover heat value, cooking chemicals and other dissolved products such as organic acids, sugars, furfural, levulinic acid, formic acid, lactic acid, and HMF. The solid phase is subjected to subsequent washing and disintegration to free solid from spent liquor and produce cellulose fibers.

A schematic representation of a cooking process (or fractionation process) of lignocellulosic biomass is shown in FIG. 1. The biomass is cooked in a digestor (reactor), and then it is subsequently washed with washing liquors as shown in FIG. 1. Cellulose fibers are prepared after defibrillation, disintegration, and screening of the cooked biomass. FIG. 1 also shows other streams of the cooking/fractionation process. The spent liquor mostly contains hemicellulose and lignin, while the washing liquor mainly contains lignin and some hemicellulose as well as “fines” that are suspended or dissolved in the liquid phase.

As used herein in some variations, “fines” are defined as small particles passing through 200 mesh (or 76 μm in diameter) screen, according to Tappi 261 cm-10, which is incorporated by reference herein. These particles may include both cellulosic and non-cellulosic materials. The fines from annual plants are mostly originated from different small vessel elements such as tracheids, parenchyma cells, etc. and called “primary fines.” The fines generated during chemical pulping of wood are mostly as a result of refining and are called “secondary fines.” Therefore the fines generated during a cooking/fractionation process (such as that depicted in FIG. 1) may be either (or both) cellulosic or non-cellulosic in origin. Generally speaking, fine components may include (but are not limited to) cellulose, hemicellulose, lignin, ash, dirt, dust, metals, and foreign materials (i.e. materials that were not in the original biomass).

Interestingly, it has been found that the lignin and mineral content of cellulose fibers decreases when the amount of washing water used is increased during the washing procedure. In order to further investigate this behavior, the amount of washing water used during water washing cycle following lignin washing cycle was altered as shown in Table 1.

Four identical cooks with sugarcane straw were conducted in a digestor. The subsequent washing procedure is almost the same for all four cooks, the only difference being the amount of washing water used in the final washing stage. Chemical composition of straw used for cooks is summarized in Table 2. Chemical composition of cellulose fibers obtained following washing procedure suggested in Table 1 is summarized in Table 3.

According to Table 3, the water washing cycle following lignin washing cycle has a significant effect on the fiber yield and lignin content (Kappa number) as well as on cellulose and hemicellulose content. A high yield of 51% (Experiment #1) was obtained without water washing cycle, while the lowest yield (Experiment #4) of 34% is measured after extensive water washing cycle. The material loss during extensive water washing step is most likely due to loss of small particles with wash water. It should be noted that during washing procedure, filtering bags with 200 mesh were used to separate solid and liquid phases by filtration. The particles smaller than 200 mesh can go through filtering bags.

The fines content of pulp produced from sugarcane straws with washing procedure described by Experiment #3 in Table 1 can be quantified by Tappi method 261 cm-90 with Britt jar assembly. The Britt Jar is a single screen classifier with 200 mesh screen or a round hole of 76 μm in diameter. Fibers are retained while fines pass through the 200 mesh screen. The fibers and fines isolated from the pulp are shown in FIG. 2. The fines content of pulp based on o.d. (oven dried) pulp is around 23%. The amount of fines (23%) determined by Britt Jar is reasonable when compared with pulp produced from hardwoods and other annual plants. The Britt jar fines reported for variety of pulp produced from hardwoods is between 5% and 21%, depending on wood species and operation conditions (Nanko et al., 2005). Sadovnik et al. (2007) reported that the pulp produced from sugarcane bagasse has high content of fines from 35 to 40%. It is also reported that bleachable-grade wheat straw pulp produced by soda pulping process in continuous Pandia digestor has 21.4% of fines (Casey, 1980).

It is clear from Table 3 that increased amount of water used during water washing cycle (as seen in Table 1) decreases lignin and ash content of cellulose fiber while increases cellulose content of fiber. Without being limited by any particular hypothesis, this reduction in lignin and ash content may be attributed to removal of fines which have high lignin and ash content and relatively low cellulose content compared to cellulose fiber during washing procedure. Fines were separated during water washing stage and chemical characterization of fines was determined. As shown in FIG. 3, the fines were allowed to pass through 200 mesh filtering bag into washing water and then they were isolated from washing water by filtering or freeze drying following centrifugation. The fines isolated by filtration and freeze-drying are shown in FIG. 3.

TABLE 1 Effect of water washing procedure on fiber yield and kappa number. Water Exp. # Water washing procedure following lignin wash Amount 1 No water washing following EtOH washing after of cooking washing 2 Just rinsed with little water following EtOH water after cooking 3 Washed with some water following EtOH washing after cooking 4 Washed with plenty of water following EtOH washing after cooking

TABLE 2 The chemical composition of sugarcane straws based on o.d. straws. Component % Arabinan 2.8 ± 0.0 Galactan 1.1 ± 0.0 Glucan 37.1 ± 0.5  Xylan 20.0 ± 0.2  Mannan 0.6 ± 0.0 Acetyl Groups 1.1 ± 0.0 Uronic Acid Groups 2.2 ± 0.3 Lignin 23.7 ± 0.2  Ash 6.8 ± 0.4 Extractives 2.6 ± 0.0 Total 98.0 ± 1.6 

TABLE 3 Effect of water washing procedure on fiber composition. % on straw % on fiber Exp.# Yield Cellulose Hemicellulose Lignin Ash Kappa # 1 50.9 73.8 5.2 7.9 11.4 52.1 2 43.3 78.8 4.4 5.9 9.3 39.2 3 38.9 81.3 4.4 3.7 8.4 25.0 4 34.4 83.5 4.6 2.8 7.4 19.5

The chemical composition of freeze-dried fines was determined and is summarized in Table 4. It is apparent from Table 4 that fines isolated during water washing stage mostly contain cellulose, lignin, and ash. Lignin, ash, and hemicellulose content of fines are each higher than the corresponding content in the fiber (see Table 3, Experiments 2, 3 and 4) while the opposite is true for cellulose. Therefore, it can be concluded that depending on the amount of fines removed during the washing stage, the cellulose content of fibers can be increased while hemicellulose, lignin, and ash content of the fiber can be reduced.

TABLE 4 Sugar and lignin contents of “fines” isolated during water washing process following AVAP ® cooking of the straws and ethanol washing stage. Component % on od “fines” Arabinan 0.7 Galactan 0.2 Glucan 57.8 Xylan 6.0 Mannan 0.2 Lignin 15.6 Ash 19.0 Total 99.5

As explained earlier, lignin content of fines is much higher than that of final pulp. This high lignin content of cellulosic fines may be a result of lignin precipitation on fines during cooking process or subsequent washing process due to higher mobility and surface area of fines in comparison to fibers (Gess, 1998). For this reason, in order to investigate the precipitation of lignin, scanning electron microscopy (SEM) images of both pulp and fines were taken and are displayed in FIGS. 4 and 5 respectively. More lignin precipitation on cellulosic fines can be observed and also it appears that pulp fibers are smoother than fines.

A schematic representation of an improved washing procedure to produce cellulose fibers (pulp) with low Kappa number and ash content, along while high cellulose content, following cooking of hardwood and/or annual plants is shown in FIG. 6. Since fines have high lignin and ash content, Kappa number (lignin content) and ash content of cellulose fibers can be lowered based on how much fines are removed during the washing procedure.

Some variations provide a process for fractionating lignocellulosic biomass, the process comprising:

(a) digesting a lignocellulosic biomass feedstock under effective conditions in the presence of a solvent for lignin, an acid or acid precursor, and water, to produce cellulose-rich solids in a digestor liquor;

(b) separating the cellulose-rich solids from the digestor liquor and washing the cellulose-rich solids with a first wash liquid comprising a wash solvent for lignin, to generate first washed cellulose-rich solids;

(c) washing the first washed cellulose-rich solids with a second wash liquid comprising water, to generate second washed cellulose-rich solids and a wash liquid comprising fines, wherein the wash liquid is introduced to or in contact with a classifier to remove at least a portion of the fines in a liquid fines-containing stream;

(d) recovering the second washed cellulose-rich solids; and

(e) optionally separating the fines from the fines-containing stream and recycling water to step (c).

In some embodiments, the classifier comprises a screen with mesh size in the range of 10 to 500. In certain embodiments, the classifier comprises a screen with mesh size in the range of 100 to 325, such as 150 to 250. In a particular embodiment, the classifier comprises a screen with mesh size of 200. Other screen sizes may be employed.

In some embodiments, the classifier is a batch or continuous centrifuge or hydrocyclone operated to remove fines within one or more selected size ranges. In certain embodiments, both a centrifuge and screen(s) may be used, such as screening the liquid discharge of a decanting centrifuge. Screen centrifuges, wherein the centrifugal acceleration allows the liquid to pass through a screen, include screen/scroll centrifuges, pusher centrifuges, peeler centrifuges, and decanter centrifuges, in which there is no physical separation between the solid and liquid phase, rather an accelerated settling due to centrifugal acceleration. Solid bowl centrifuges or conical plate centrifuges may also be employed.

Dispersers may also be added to liberate more fines if necessary. During step (b) and/or step (c), a disperser may be utilized to liberate fines from the second washed cellulose-rich solids. A disperser may liberate additional fines that would not have otherwise been released. In some embodiments, a disperser is a simple mixing tank, i.e. a stirred tank or vessel. Dispersers may also be in-line (static) mixers, high-shear mixers, centrifuges, or other equipment. In some embodiments, the disperser is integrated with the classifier; for example, a centrifuge may be adapted to both disperse fines from solids as well as classify the fines as described above.

Instead of a disperser, or in addition, other reagents may also be used to liberate more fines and/or remove minerals remaining in the pulp at this stage, depending on targeted quality of product. During step (b) and/or step (c), one or more additives may be introduced to remove minerals remaining in the first washed cellulose-rich solids and/or the second washed cellulose-rich solids. Additives include, but are not limited to, acids, bases, salts, carbon (such as activated carbon or carbon foams), metal foams, silica, alumina, or other compounds.

Cellulose fibers may also be bleached to remove remaining lignin from the fiber. Any known bleaching sequence may be utilized.

The process may be continuous, semi-continuous, or batch. In some embodiments, one or more steps are conducted countercurrently. In certain embodiments, the process is batch or semi-continuous, washing is conducted in simulated countercurrent fashion, and multiple wash streams (such as two, three, or more wash streams) are generated.

In some embodiments, the solvent for lignin includes an aliphatic alcohol, such as ethanol. Preferably, the process further comprises recycling the solvent for lignin back to the digestor. Also, the process preferably comprises recycling the unreacted acid or acid precursor to the digestor.

In some embodiments, the acid catalyst (or acid precursor) is a sulfur-containing compound or a derivative thereof. For example the sulfur-containing compound may be selected from the group consisting of sulfur dioxide, sulfur trioxide, sulfurous acid, sulfuric acid, sulfonic acids, lignosulfonic acids, elemental sulfur, and combinations thereof.

In some embodiments, the acid catalyst is a nitrogen-containing compound (e.g., HNO₃) or a derivative thereof. In some embodiments, the acid catalyst is a phosphorous-containing compound (e.g., H₃PO₄) or a derivative thereof. In some embodiments, the acid catalyst is one or more hydrogen halides (e.g., HBr or HCl).

Removal of SO₂ may be conducted in a sulfur dioxide separation unit selected from the group consisting of a flash vessel, a stripping column, a distillation column, and combinations thereof, operated under vacuum or pressure. In some embodiments, the sulfur dioxide separation unit is a stripping column employing steam for stripping the unreacted sulfur dioxide.

The process may further include dilution with liquid water during one or more steps. As intended here, dilution with liquid water may occur via injection of a liquid-phase stream comprising water, which may be fresh water or recycled water (e.g., process condensate); alternatively, or additionally, dilution with liquid water may occur via injection of steam which condenses to form liquid water that dilutes a process stream. Dilution with liquid water may assist in the precipitating at least some of the lignin in a lignin-containing stream.

In some embodiments, the process further comprises pH adjustment during one or more steps. The pH adjustment may assist in controlling lignin precipitation in the lignin-containing stream. For example, raising pH may increase lignin solubility in aqueous solution, while lowering pH may reduce lignin solubility in aqueous solution, in some embodiments.

In embodiments employing SO₂ during fractionation, some amount of lignin sulfonation typically occurs. In some embodiments, the process comprises further lignin sulfonation during one or more steps. Lignin sulfonation generally increases lignin solubility in aqueous solution. Lignin sulfonation may be accomplished by reaction of soluble lignin or suspended lignin with SO₂ or another sulfur-containing compound.

The lignin-containing stream may be in various forms and phases, including multiple phases (two, three, or more). For example, the lignin-containing stream may be in the form of a slurry. In certain embodiments, lignin-containing stream or product contains lignin in substantially solid form, such as lignin solids recovered periodically from a semi-continuous process or lignin solids that form a filter cake.

In some embodiments, the lignin-containing stream contains colloids of lignin dispersed in the continuous phase (liquor). Colloids of lignin may be removed by filtration or centrifugation, for example. To enhance the removal of lignin colloids from suspension, it may be desirable to adjust the pH of the suspension either during or after dilution with water. Also, additives may be introduced to change kinetics or thermodynamics of colloid phase formation. In some embodiments, the lignin/lignosulfonate ratio is optimized during digestion or downstream, to adjust the properties of the colloidal suspension.

The hemicelluloses may be recovered for fermentation or for further processing. In some embodiments, the process further comprises a step of hemicellulose hydrolysis with an acid or enzymes. The acid for hemicellulose hydrolysis may include lignosulfonic acids that are derived from the initial fractionation step.

The cellulose-rich solids may be recovered as a pulp product. Alternatively, or additionally, the cellulose-rich solids may be hydrolyzed to produce glucose.

The present invention includes apparatus and systems to carry out the processes described herein. The present invention also includes products produced by the processes described herein. Such products include biomass-derived sugars, cellulose materials, lignin, lignosulfonates, and other co-products.

As used herein, “lignocellulosic biomass” means any material containing cellulose and lignin. Lignocellulosic biomass may also contain hemicellulose. Mixtures of one or more types of biomass can be used. In some embodiments, the biomass feedstock comprises both a lignocellulosic component (such as one described above) in addition to a sucrose-containing component (e.g., sugarcane or energy cane) and/or a starch component (e.g., corn, wheat, rice, etc.).

The biomass feedstock may be selected from hardwoods, softwoods, forest residues, industrial wastes, pulp and paper wastes, consumer wastes, or combinations thereof. Some embodiments utilize agricultural residues, which include lignocellulosic biomass associated with food crops, annual grasses, energy crops, or other annually renewable feedstocks. Exemplary agricultural residues include, but are not limited to, corn stover, corn fiber, wheat straw, sugarcane bagasse, sugarcane straw, rice straw, oat straw, barley straw, miscanthus, energy cane straw/residue, or combinations thereof. In some embodiments, the biomass feedstock is not softwood.

Various moisture levels may be associated with the starting biomass. The biomass feedstock need not be, but may be, relatively dry. In general, the biomass is in the form of a particulate or chip, but particle size is not critical in this invention.

Reaction conditions and operation sequences may vary widely. Some embodiments employ conditions described in U.S. Pat. No. 8,030,039, issued Oct. 4, 2011; U.S. Pat. No. 8,038,842, issued Oct. 11, 2011; U.S. Pat. No. 8,268,125, issued Sep. 18, 2012; and U.S. patent application Ser. Nos. 13/004,431; 12/234,286; 13/585,710; 12/250,734; 12/397,284; 12/304,046; 13/500,916; 13/626,220; 12/854,869; 61/732,047; 61/735,738; 61/739,343; 61/747,010; 61/747,105; 61/747,376; 61/747,379; 61/747,382; and 61/747,408 including the prosecution histories thereof. Each of these commonly owned patent applications is hereby incorporated by reference herein in its entirety. In some embodiments, the process is a variation of the AVAP® process technology which is commonly owned with the assignee of this patent application.

In some embodiments, a first process step is “cooking” (equivalently, “digesting”) which fractionates the three lignocellulosic material components (cellulose, hemicellulose, and lignin) to allow easy downstream removal. Specifically, hemicelluloses are dissolved and over 50% are completely hydrolyzed; cellulose is separated but remains resistant to hydrolysis; and part of the lignin is sulfonated into water-soluble lignosulfonates.

The lignocellulosic material is processed in a solution (cooking liquor) of solvent, water, and sulfur dioxide. The cooking liquor preferably contains at least 10 wt %, such as at least 20 wt %, 30 wt %, 40 wt %, or 50 wt % of a solvent for lignin. By “solvent for lignin,” it is meant a chemical that is capable of dissolving at least some lignin, in native (non-sulfonated) form, at the conditions of digestion.

For example, the cooking liquor may contain about 30-70 wt % solvent, such as about 50 wt % solvent. The solvent for lignin may be an aliphatic alcohol, such as methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, isobutanol, 1-pentanol, 1-hexanol, or cyclohexanol. The solvent for lignin may be an aromatic alcohol, such as phenol or cresol. Other lignin solvents are possible, such as (but not limited to) glycerol, methyl ethyl ketone, or diethyl ether. Combinations of more than one solvent may be employed.

Preferably, enough solvent is included in the extractant mixture to dissolve the lignin present in the starting material. The solvent for lignin may be completely miscible, partially miscible, or immiscible with water, so that there may be more than one liquid phase. Potential process advantages arise when the solvent is miscible with water, and also when the solvent is immiscible with water. When the solvent is water-miscible, a single liquid phase forms, so mass transfer of lignin and hemicellulose extraction is enhanced, and the downstream process must only deal with one liquid stream. When the solvent is immiscible in water, the extractant mixture readily separates to form liquid phases, so a distinct separation step can be avoided or simplified. This can be advantageous if one liquid phase contains most of the lignin and the other contains most of the hemicellulose sugars, as this facilitates recovering the lignin from the hemicellulose sugars.

The cooking liquor preferably contains sulfur dioxide and/or sulfurous acid (H₂SO₃). The cooking liquor preferably contains SO₂, in dissolved or reacted form, in a concentration of at least 3 wt %, preferably at least 6 wt %, more preferably at least 8 wt %, such as about 9 wt %, 10 wt %, 11 wt %, 12 wt %, 13 wt %, 14 wt %, 15 wt %, 20 wt %, 25 wt %, 30 wt % or higher. The cooking liquor may also contain one or more species, separately from SO₂, to adjust the pH. The pH of the cooking liquor is typically about 4 or less.

Sulfur dioxide is a preferred acid catalyst, because it can be recovered easily from solution after hydrolysis. The majority of the SO₂ from the hydrolysate may be stripped and recycled back to the reactor. Recovery and recycling translates to less lime required compared to neutralization of comparable sulfuric acid, less solids to dispose of, and less separation equipment. The increased efficiency owing to the inherent properties of sulfur dioxide mean that less total acid or other catalysts may be required. This has cost advantages, since sulfuric acid can be expensive. Additionally, and quite significantly, less acid usage also will translate into lower costs for a base (e.g., lime) to increase the pH following hydrolysis, for downstream operations. Furthermore, less acid and less base will also mean substantially less generation of waste salts (e.g., gypsum) that may otherwise require disposal.

The cooking is performed in one or more stages using batch or continuous digestors. Solid and liquid may flow cocurrently or countercurrently, or in any other flow pattern that achieves the desired fractionation. The cooking reactor may be internally agitated, if desired.

Depending on the lignocellulosic material to be processed, the cooking conditions are varied, with temperatures from about 65° C. to 175° C., for example 75° C., 85° C., 95° C., 105° C., 115° C., 125° C., 130° C., 135° C., 140° C., 145° C., 150° C., 155° C., 165° C. or 170° C., and corresponding pressures from about 1 atmosphere to about 15 atmospheres in the liquid or vapor phase. The cooking time of one or more stages may be selected from about 15 minutes to about 720 minutes, such as about 30, 45, 60, 90, 120, 140, 160, 180, 250, 300, 360, 450, 550, 600, or 700 minutes. Generally, there is an inverse relationship between the temperature used during the digestion step and the time needed to obtain good fractionation of the biomass into its constituent parts.

The cooking liquor to lignocellulosic material ratio may be selected from about 1 to about 10, such as about 2, 3, 4, 5, or 6. In some embodiments, biomass is digested in a pressurized vessel with low liquor volume (low ratio of cooking liquor to lignocellulosic material), so that the cooking space is filled with ethanol and sulfur dioxide vapor in equilibrium with moisture. The cooked biomass is washed in alcohol-rich solution to recover lignin and dissolved hemicelluloses, while the remaining pulp is further processed. In some embodiments, the process of fractionating lignocellulosic material comprises vapor-phase cooking of lignocellulosic material with aliphatic alcohol (or other solvent for lignin), water, and sulfur dioxide. See, for example, U.S. Pat. Nos. 8,038,842 and 8,268,125 which are incorporated by reference herein.

A portion or all of the sulfur dioxide may be present as sulfurous acid in the extract liquor. In certain embodiments, sulfur dioxide is generated in situ by introducing sulfurous acid, sulfite ions, bisulfite ions, combinations thereof, or a salt of any of the foregoing. Excess sulfur dioxide, following hydrolysis, may be recovered and reused.

In some embodiments, sulfur dioxide is saturated in water (or aqueous solution, optionally with an alcohol) at a first temperature, and the hydrolysis is then carried out at a second, generally higher, temperature. In some embodiments, sulfur dioxide is sub-saturated. In some embodiments, sulfur dioxide is super-saturated. In some embodiments, sulfur dioxide concentration is selected to achieve a certain degree of lignin sulfonation, such as 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, or 10% sulfur content. SO₂ reacts chemically with lignin to form stable lignosulfonic acids which may be present both in the solid and liquid phases.

The concentration of sulfur dioxide, additives, and aliphatic alcohol (or other solvent) in the solution and the time of cook may be varied to control the yield of cellulose and hemicellulose in the pulp. The concentration of sulfur dioxide and the time of cook may be varied to control the yield of lignin versus lignosulfonates in the hydrolysate. In some embodiments, the concentration of sulfur dioxide, temperature, and the time of cook may be varied to control the yield of fermentable sugars.

Once the desired amount of fractionation of both hemicellulose and lignin from the solid phase is achieved, the liquid and solid phases are separated. Conditions for the separation may be selected to minimize the reprecipitation of the extracted lignin on the solid phase. This is favored by conducting separation or washing at a temperature of at least the glass-transition temperature of lignin (about 120° C.).

The physical separation can be accomplished either by transferring the entire mixture to a device that can carry out the separation and washing, or by removing only one of the phases from the reactor while keeping the other phase in place. The solid phase can be physically retained by appropriately sized screens through which liquid can pass. The solid is retained on the screens and can be kept there for successive solid-wash cycles. Alternately, the liquid may be retained and solid phase forced out of the reaction zone, with centrifugal or other forces that can effectively transfer the solids out of the slurry. In a continuous system, countercurrent flow of solids and liquid can accomplish the physical separation.

The recovered solids normally will contain a quantity of lignin and sugars, some of which can be removed easily by washing. The washing-liquid composition can be the same as or different than the liquor composition used during fractionation. Multiple washes may be performed to increase effectiveness. Preferably, one or more washes are performed with a composition including a solvent for lignin, to remove additional lignin from the solids, followed by one or more washes with water to displace residual solvent and sugars from the solids, as well as release fines from the fibers as disclosed in detail herein. Recycle streams, such as from solvent-recovery operations, may be used to wash the solids.

After separation and washing as described, a solid phase and at least one liquid phase are obtained. The solid phase contains substantially undigested cellulose. A single liquid phase is usually obtained when the solvent and the water are miscible in the relative proportions that are present. In that case, the liquid phase contains, in dissolved form, most of the lignin originally in the starting lignocellulosic material, as well as soluble monomeric and oligomeric sugars formed in the hydrolysis of any hemicellulose that may have been present. Multiple liquid phases tend to form when the solvent and water are wholly or partially immiscible. The lignin tends to be contained in the liquid phase that contains most of the solvent. Hemicellulose hydrolysis products tend to be present in the liquid phase that contains most of the water.

In some embodiments, hydrolysate from the cooking step is subjected to pressure reduction. Pressure reduction may be done at the end of a cook in a batch digestor, or in an external flash tank after extraction from a continuous digestor, for example. The flash vapor from the pressure reduction may be collected into a cooking liquor make-up vessel. The flash vapor contains substantially all the unreacted sulfur dioxide which may be directly dissolved into new cooking liquor. The cellulose is then removed to be washed and further treated as desired.

A process washing step recovers the hydrolysate from the cellulose. The washed cellulose is pulp that may be used for various purposes (e.g., paper or nanocellulose production). The weak hydrolysate from the washer continues to the final reaction step; in a continuous digestor this weak hydrolysate may be combined with the extracted hydrolysate from the external flash tank. In some embodiments, washing and/or separation of hydrolysate and cellulose-rich solids is conducted at a temperature of at least about 100° C., 110° C., or 120° C. The washed cellulose may also be used for glucose production via cellulose hydrolysis with enzymes or acids.

In another reaction step, the hydrolysate may be further treated in one or multiple steps to hydrolyze the oligomers into monomers. This step may be conducted before, during, or after the removal of solvent and sulfur dioxide. The solution may or may not contain residual solvent (e.g. alcohol). In some embodiments, sulfur dioxide is added or allowed to pass through to this step, to assist hydrolysis. In these or other embodiments, an acid such as sulfurous acid or sulfuric acid is introduced to assist with hydrolysis. In some embodiments, the hydrolysate is autohydrolyzed by heating under pressure. In some embodiments, no additional acid is introduced, but lignosulfonic acids produced during the initial cooking are effective to catalyze hydrolysis of hemicellulose oligomers to monomers. In various embodiments, this step utilizes sulfur dioxide, sulfurous acid, sulfuric acid at a concentration of about 0.01 wt % to 30 wt %, such as about 0.05 wt %, 0.1 wt %, 0.2 wt %, 0.5 wt %, 1 wt %, 2 wt %, 5 wt %, 10 wt %, or 20 wt %. This step may be carried out at a temperature from about 100° C. to 220° C., such as about 110° C., 120° C., 130° C., 140° C., 150° C., 160° C., 170° C., 180° C., 190° C., 200° C., or 210° C. Heating may be direct or indirect to reach the selected temperature.

The reaction step produces fermentable sugars which can then be concentrated by evaporation to a fermentation feedstock. Concentration by evaporation may be accomplished before, during, or after the treatment to hydrolyze oligomers. The final reaction step may optionally be followed by steam stripping of the resulting hydrolysate to remove and recover sulfur dioxide and alcohol, and for removal of potential fermentation-inhibiting side products. The evaporation process may be under vacuum or pressure, from about −0.1 atmospheres to about 10 atmospheres, such as about 0.1 atm, 0.3 atm, 0.5 atm, 1.0 atm, 1.5 atm, 2 atm, 4 atm, 6 atm, or 8 atm.

Recovering and recycling the sulfur dioxide may utilize separations such as, but not limited to, vapor-liquid disengagement (e.g. flashing), steam stripping, extraction, or combinations or multiple stages thereof. Various recycle ratios may be practiced, such as about 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 0.95, or more. In some embodiments, about 90-99% of initially charged SO₂ is readily recovered by distillation from the liquid phase, with the remaining 1-10% (e.g., about 3-5%) of the SO₂ primarily bound to dissolved lignin in the form of lignosulfonates.

In a preferred embodiment, the evaporation step utilizes an integrated alcohol stripper and evaporator. Evaporated vapor streams may be segregated so as to have different concentrations of organic compounds in different streams. Evaporator condensate streams may be segregated so as to have different concentrations of organic compounds in different streams. Alcohol may be recovered from the evaporation process by condensing the exhaust vapor and returning to the cooking liquor make-up vessel in the cooking step. Clean condensate from the evaporation process may be used in the washing step.

In some embodiments, an integrated alcohol stripper and evaporator system is employed, wherein aliphatic alcohol is removed by vapor stripping, the resulting stripper product stream is concentrated by evaporating water from the stream, and evaporated vapor is compressed using vapor compression and is reused to provide thermal energy.

The hydrolysate from the evaporation and final reaction step contains mainly fermentable sugars but may also contain lignin depending on the location of lignin separation in the overall process configuration. The hydrolysate may be concentrated to a concentration of about 5 wt % to about 60 wt % solids, such as about 10 wt %, 15 wt %, 20 wt %, 25 wt %, 30 wt %, 35 wt %, 40 wt %, 45 wt %, 50 wt % or 55 wt % solids. The hydrolysate contains fermentable sugars.

Fermentable sugars are defined as hydrolysis products of cellulose, galactoglucomannan, glucomannan, arabinoglucuronoxylans, arabinogalactan, and glucuronoxylans into their respective short-chained oligomers and monomer products, i.e., glucose, mannose, galactose, xylose, and arabinose. The fermentable sugars may be recovered in purified form, as a sugar slurry or dry sugar solids, for example. Any known technique may be employed to recover a slurry of sugars or to dry the solution to produce dry sugar solids.

In some embodiments, the fermentable sugars are fermented to produce biochemicals or biofuels such as (but by no means limited to) ethanol, isopropanol, acetone, 1-butanol, isobutanol, lactic acid, succinic acid, or any other fermentation products. Some amount of the fermentation product may be a microorganism or enzymes, which may be recovered if desired.

When the fermentation will employ bacteria, such as Clostridia bacteria, it is preferable to further process and condition the hydrolysate to raise pH and remove residual SO₂ and other fermentation inhibitors. The residual SO₂ (i.e., following removal of most of it by stripping) may be catalytically oxidized to convert residual sulfite ions to sulfate ions by oxidation. This oxidation may be accomplished by adding an oxidation catalyst, such as FeSO4.7H₂O, that oxidizes sulfite ions to sulfate ions. Preferably, the residual SO₂ is reduced to less than about 100 ppm, 50 ppm, 25 ppm, 10 ppm, 5 ppm, or 1 ppm.

The process fermentation and distillation steps are intended for the production of fermentation products, such as alcohols or organic acids. After removal of cooking chemicals and lignin, and further treatment (oligomer hydrolysis), the hydrolysate contains mainly fermentable sugars in water solution from which any fermentation inhibitors have been preferably removed or neutralized. The hydrolysate is fermented to produce dilute alcohol or organic acids, from 1 wt % to 20 wt % concentration. The dilute product is distilled or otherwise purified as is known in the art.

When alcohol is produced, such as ethanol, some of it may be used for cooking liquor makeup in the process cooking step. Also, in some embodiments, a distillation column stream, such as the bottoms, with or without evaporator condensate, may be reused to wash cellulose. In some embodiments, lime may be used to dehydrate product alcohol. Side products may be removed and recovered from the hydrolysate. These side products may be isolated by processing the vent from the final reaction step and/or the condensate from the evaporation step. Side products include furfural, hydroxymethyl furfural (HMF), methanol, acetic acid, and lignin-derived compounds, for example.

The cellulose-rich material is highly reactive in the presence of industrial cellulase enzymes that efficiently break the cellulose down to glucose monomers. It has been found experimentally that the cellulose-rich material, which generally speaking is highly delignified, rapidly hydrolyzes to glucose with relatively low quantities of enzymes. For example, the cellulose-rich solids may be converted to glucose with at least 80% yield within 24 hours at 50° C. and 2 wt % solids, in the presence of a cellulase enzyme mixture in an amount of no more than 15 filter paper units (FPU) per g of the solids. In some embodiments, this same conversion requires no more than 5 FPU per g of the solids.

The glucose may be fermented to an alcohol, an organic acid, or another fermentation product. The glucose may be used as a sweetener or isomerized to enrich its fructose content. The glucose may be used to produce baker's yeast. The glucose may be catalytically or thermally converted to various organic acids and other materials.

In some embodiments, the cellulose-rich material is further processed into one more cellulose products. Cellulose products include market pulp, dissolving pulp (also known as α-cellulose), fluff pulp, purified cellulose, paper, paper products, and so on. Further processing may include bleaching, if desired. Further processing may include modification of fiber length or particle size, such as when producing nanocellulose or nanofibrillated or microfibrillated cellulose. It is believed that the cellulose produced by this process is highly amenable to derivatization chemistry for cellulose derivatives and cellulose-based materials such as polymers.

When hemicellulose is present in the starting biomass, all or a portion of the liquid phase contains hemicellulose sugars and soluble oligomers. It is preferred to remove most of the lignin from the liquid, as described above, to produce a fermentation broth which will contain water, possibly some of the solvent for lignin, hemicellulose sugars, and various minor components from the digestion process. This fermentation broth can be used directly, combined with one or more other fermentation streams, or further treated. Further treatment can include sugar concentration by evaporation; addition of glucose or other sugars (optionally as obtained from cellulose saccharification); addition of various nutrients such as salts, vitamins, or trace elements; pH adjustment; and removal of fermentation inhibitors such as acetic acid and phenolic compounds. The choice of conditioning steps should be specific to the target product(s) and microorganism(s) employed.

In some embodiments, hemicellulose sugars are not fermented but rather are recovered and purified, stored, sold, or converted to a specialty product. Xylose, for example, can be converted into xylitol.

Lignin produced in accordance with the invention can be used as a fuel. As a solid fuel, lignin is similar in energy content to coal. Lignin can act as an oxygenated component in liquid fuels, to enhance octane while meeting standards as a renewable fuel. The lignin produced herein can also be used as polymeric material, and as a chemical precursor for producing lignin derivatives. The sulfonated lignin may be sold as a lignosulfonate product, or burned for fuel value. In certain embodiments, the process further comprises combusting or gasifying the sulfonated lignin, recovering sulfur contained in the sulfonated lignin in a gas stream comprising reclaimed sulfur dioxide, and then recycling the reclaimed sulfur dioxide for reuse.

Native (non-sulfonated) lignin is hydrophobic, while lignosulfonates are hydrophilic. Hydrophilic lignosulfonates may have less propensity to clump, agglomerate, and stick to surfaces. Even lignosulfonates that do undergo some condensation and increase of molecular weight, will still have an HSO₃ group that will contribute some solubility (hydrophilic). After the evaporative precipitation or other method to remove water-insoluble lignin, the remaining water-soluble lignosulfonates may be precipitated by converting the hydrolysate to an alkaline condition (pH higher than 7) using, for example, an alkaline earth oxide, preferably calcium oxide (lime). The lignosulfonate precipitate may be filtered. The lignosulfonate filter cake may be dried as a co-product or burned or gasified for energy production. The hydrolysate from filtering may be recovered and sold as a concentrated sugar solution product or further processed in a subsequent fermentation or other reaction step.

Lignin with specific property ranges may be obtained by doing a multiple-effect evaporative crystallization to purposely create lignin precipitates with various properties. Thus in some embodiments, several types of non-sulfonated lignin or lignin with low levels of sulfur may be obtained, in addition to one or more sulfonated lignins.

The present invention also provides systems configured for carrying out the disclosed processes, and compositions produced therefrom. Any stream generated by the disclosed processes may be partially or completed recovered, purified or further treated, and/or marketed or sold.

In this detailed description, reference has been made to multiple embodiments of the invention and non-limiting examples relating to how the invention can be understood and practiced. Other embodiments that do not provide all of the features and advantages set forth herein may be utilized, without departing from the spirit and scope of the present invention. This invention incorporates routine experimentation and optimization of the methods and systems described herein. Such modifications and variations are considered to be within the scope of the invention defined by the claims.

All publications, patents, and patent applications cited in this specification are herein incorporated by reference in their entirety as if each publication, patent, or patent application were specifically and individually put forth herein.

Where methods and steps described above indicate certain events occurring in certain order, those of ordinary skill in the art will recognize that the ordering of certain steps may be modified and that such modifications are in accordance with the variations of the invention. Additionally, certain of the steps may be performed concurrently in a parallel process when possible, as well as performed sequentially.

Therefore, to the extent there are variations of the invention, which are within the spirit of the disclosure or equivalent to the inventions found in the appended claims, it is the intent that this patent will cover those variations as well. The present invention shall only be limited by what is claimed.

REFERENCES

-   Casey, J. P. Pulp and paper, chemistry and chemical technology. 3rd     edition, vol. 1, John Wily and Sons, Inc., USA, 1980. -   Gess, J. M. Retention of fines and fillers during papermaking, Tappi     Press. Atlanta, Ga. (1998). -   Nanko, H., Button, A., Hillman, D. The world of market Pulp,     Appleton, Wis., WOMP LLC (2005). -   Sadovnik, J. C, Betancourt, L. A, Ramos, J., and Alban, F., “Fines     Study in the Bleached Pulp from the Sugar Cane Bagasse,” Tappi 2007     Engineering, Pulping & Environmental Conference (2007). -   Tappi standard T 261 cm-90 “Fines fraction of paper stock by wet     screening,” Tappi press, Atlanta, Ga., USA (1992). 

What is claimed is:
 1. A process for fractionating lignocellulosic biomass, said process comprising: (a) digesting a lignocellulosic biomass feedstock under effective conditions in the presence of a solvent for lignin, an acid or acid precursor, and water, to produce cellulose-rich solids in a digestor liquor; (b) separating said cellulose-rich solids from said digestor liquor and washing said cellulose-rich solids with a first wash liquid comprising a wash solvent for lignin, to generate first washed cellulose-rich solids; (c) washing said first washed cellulose-rich solids with a second wash liquid comprising water, to generate second washed cellulose-rich solids and a wash liquor comprising fines, wherein said wash liquor is introduced to or in contact with a classifier to remove at least a portion of said fines in a liquid fines-containing stream; (d) recovering said second washed cellulose-rich solids; and (e) optionally separating said fines from said fines-containing stream and recycling water contained in said fines-containing stream back to step (c).
 2. The process of claim 1, wherein said lignocellulosic biomass feedstock is a hardwood.
 3. The process of claim 1, wherein said lignocellulosic biomass feedstock is an annual plant or agricultural residue.
 4. The process of claim 1, wherein said solvent for lignin is ethanol.
 5. The process of claim 1, wherein said solvent for lignin is the same as said wash solvent for lignin.
 6. The process of claim 1, wherein said acid or acid precursor is sulfur dioxide.
 7. The process of claim 1, wherein said classifier comprises a screen with mesh size in the range of 10 to
 500. 8. The process of claim 7, wherein said classifier comprises a screen with mesh size in the range of 150 to
 250. 9. The process of claim 1, wherein during step (b) and/or step (c), a disperser is utilized to liberate said fines from said second washed cellulose-rich solids.
 10. The process of claim 1, wherein during step (c), at least 95% of said fines contained in said wash liquor are removed into said liquid fines-containing stream.
 11. The process of claim 1, wherein during step (b) and/or step (c), one or more additives are introduced to remove minerals remaining in said first washed cellulose-rich solids and/or said second washed cellulose-rich solids.
 12. The process of claim 1, wherein said second washed cellulose-rich solids have a lower Kappa number compared to cellulose-rich solids from an otherwise-identical process without a classifier to remove at least a portion of said fines.
 13. The process of claim 1, wherein said second washed cellulose-rich solids have a lower ash content compared to cellulose-rich solids from an otherwise-identical process without a classifier to remove at least a portion of said fines.
 14. The process of claim 1, wherein said second washed cellulose-rich solids have a lower hemicellulose content compared to cellulose-rich solids from an otherwise-identical process without a classifier to remove at least a portion of said fines.
 15. The process of claim 1, wherein said second washed cellulose-rich solids contain about 75% or more cellulose, about 7 wt % or less lignin, about 5 wt % or less hemicellulose, and about 10 wt % or less ash.
 16. The process of claim 15, wherein said second washed cellulose-rich solids contain about 80% or more cellulose, about 3 wt % or less lignin, about 5 wt % or less hemicellulose, and about 8 wt % or less ash.
 17. The process claim 1, wherein steps (a)-(c) are conducted countercurrently.
 18. The process of claim 1, wherein said process is batch or semi-continuous, wherein step (b) and/or step (c) is conducted in simulated countercurrent fashion, and wherein multiple wash streams are generated.
 19. A method of separating fines from cellulose-rich solids, said method comprising: (a) obtaining a biomass digestor liquor comprising cellulose-rich solids; (b) separating said cellulose-rich solids from said digestor liquor and washing said cellulose-rich solids with a first wash liquid, to generate first washed cellulose-rich solids; (c) washing said first washed cellulose-rich solids with a second wash liquid, to generate second washed cellulose-rich solids and a wash liquor comprising fines, wherein said wash liquor is introduced to or in contact with a classifier to remove at least a portion of said fines in a liquid fines-containing stream; (d) recovering said second washed cellulose-rich solids; and (e) optionally separating said fines from said fines-containing stream and recycling water contained in said fines-containing stream back to step (c).
 20. A method of separating fines from cellulose-rich solids, said method comprising: (a) obtaining a biomass digestor liquor comprising cellulose-rich solids; (b) separating said cellulose-rich solids from said digestor liquor and washing said cellulose-rich solids with a wash liquid, to generate washed cellulose-rich solids and a wash liquor comprising fines, wherein said wash liquor is introduced to or in contact with a classifier to remove at least a portion of said fines in a liquid fines-containing stream; (c) recovering said second washed cellulose-rich solids; and (d) optionally separating said fines from said fines-containing stream and recycling water contained in said fines-containing stream back to step (b). 